<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Test21</title>
</head>
<body>
    <script type="text/javascript">
        var a = prompt("请输入a的值");
        if (a>60 && a<=70){
            alert("b=1");
        }else if (a>70 && a<=80){
            alert("b=2");
        }else if (a>80 && a<=90){
            alert("b=3");
        }else if (a>90){
            alert("b=4");
        }else {
            alert("输入的数字必须大于60");
        }
    </script>
</body>
</html>